Digital duplicator printing is a rotary printing process in which a stencil containing pixel image voids is mounted on the circumference of a drum which contains printing ink and which is located in a printing machine. The drum circumference is a mesh screen. To make prints, sheets of paper are passed under the drum in point contact as it rotates, inks being forced through the screens and stencil on to the paper.
It is well known to use water-in-oil emulsion inks for this printing process. The inks may contain a pigment in the water or oil phase. Inks which contain pigment in the water phase can provide a number of advantages. Typically, better image density and reduced set-off are achieved in comparison with similar inks with pigment in the oil phase. For example, EP-A-778323 describes water-in-oil emulsion inks comprising carbon black in the water phase. It is claimed that by selecting carbon black having a pH value in the range 6 to 10 it is possible to achieve, amongst other properties, excellent emulsion stability. Inks containing carbon with a pH value outside this range are claimed to exhibit droplet size enlargement, phase separation and other changes that characterise instability of the emulsion. Inks undergoing these types of change become progressively more fluid.
In our investigation of such systems we have found that while it is possible to achieve inks which are resistant to the above-described changes, it is also possible for these inks to undergo an equally detrimental process of change whereby they become thicker in consistency as they age. This change is evidenced by a significant increase in rheological measures such as yield value and PPP value. PPP is a measurement of the spread of a defined volume of ink between two glass plates (Parallel Plate Plastometer). This method is described by A Voet in American Ink Maker, Volume 28, 1950.
Such changes in rheology can have an adverse effect on the performance of the ink in a digital duplicator. Typical problems include difficulty in pumping ink from the cartridge in which it is supplied, leading to a high residue of unused ink when the cartridge is discarded and a consequent increase in the cost of printing. There may also be adverse effects on printing properties such as ink consumption and image recovery, which is the number of prints required to achieve full image quality when resuming printing after a period of non-use.
A further problem which is encountered when formulating water-in-oil emulsion inks with pigment in either the water phase or the oil phase is the difficulty in adjusting ink rheology independently of the water content of the emulsion. The volume of the dispersed phase (Awater content@) of water-in-oil emulsion inks plays a critical role in determining their physical and printing properties. In particular, this parameter is of fundamental importance in determining the flow properties of the ink. Inks for use in digital duplicators require their flow properties to be controlled and limited in order to prevent the occurrence of tail flooding. Tail flooding is a printing defect whereby the layer of ink, which is held between the digital stencil and the screens on the surface of the printing drum, exudes from under the trailing edge of the stencil during the printing process. This leads to a catastrophic failure in print quality, as ink is transferred on to the impression roller and thence as spurious marks on to the back of the paper sheets being printed. Such defects due to excessive ink flow can often be remedied by increasing the water content of the ink. However in so doing, other Theological properties of the ink are changed, and in particular the ink suffers an increase in PPP and yield value. Circumstances can therefore arise where control of ink flow can only be achieved at the expense of creating very stiff paste inks, i.e., inks with very high PPP and yield values. It would, therefore, be very desirable to have the means of adjusting these parameters independently of any adjustment to the water content of the ink.
It is an objective of the present invention to achieve a water-in-oil emulsion ink comprising a dispersion of carbon pigment in the water phase which maintains stable rheology on ageing. A further objective is to provide a means whereby it is possible to adjust rheological parameters such as yield value independently of any adjustment to the water content of the ink.